The basis for induction of T cell-independent (TI) humoral immunity (HI) in response to bacterial polysaccarides (PS) is unknown. Anti-lg mAbs linked to dextran is an in vitro polyclonal model for B cell activation in response to PS antigens. Natural killer (NK) cells, through release of IFN-(gamma) and GM-CSF stimulate lg secretion by anti-lg-dextran- activated B cells and thus may regulate TI HI in a positive manner. Analysis of in vitro murine NK cell clones further suggests heterogeneity of NK cells for delivery of help and suppression, for induction of lg synthesis. A marked reduction in PS-specific lg after immunization of T + NK cell-deficient mice (CD3 (epsilon) transgenic) relative to mice deficient only in T cells (athymic nude) with the PS-encapsulated bacterium, S. pneumoniae, in contrast to a pure haptenated PS (TNP- Ficoll), strongly suggests a physiologic dependence on NK cells for TI HI, and further indicates that the nature of the TI stimulus impacts on the role of NK cells for IT induction of lg synthesis in vivo. This proposal will use anti-lg dextran in vitro and PS antigens and PS- containing bacteria in vivo to explore the role of NK cells in mediating TI HI. In vitro murine NK cells clones will be analyzed for their ability to deliver B cell help and suppression. The basis for their distinct functional phenotypes will be established through analyses of cytokine secretion profiles and expression of various cell surface proteins. In vivo studies, largely using T and/or NK cell deficient knockout and transgenic mice will establish the physiologic role of NK cells in mediating HI in response to distinct TI stimuli, and determine the features of the immunizing agent which lead to NK cell dependence for lg responses. Finally, the nature of the NK cell help for PS specific lg synthesis in vivo will be explored through analyses of cytokine induction profiles and cytokine-dependence of in vivo HI responses to various TI antigens. These studies will be the first to elucidate the pathway(s) leading to PS-specific HI and further our understanding of the host response to PS-encapsulated bacterial pathogens.